1. Field of the Invention
The present invention relates to a process for manufacturing a semiconductor device and a semiconductor device formed by the process. More particularly, it relates to a process for manufacturing a semiconductor device comprising a photoelectric conversion section, and a semiconductor device formed by the process.
2. Description of Related Art
Semiconductor image pickup devices such as Charge Coupled Device (CCD) Image sensors, Complementary Metal Oxide Semiconductor (CMOS) Image sensors and the like have been used for various applications such as digital cameras, video cameras, cellular phones with cameras, scanners, digital copiers, facsimiles and the like. As they are widely used, there have been increasing demands for reduction in size and in cost of the devices as well as better functions and performance such as increase of the pixel count and improvement of the light sensitivity of the devices.
Requirements for the reduction in cost of the devices in addition to the reduction in size of the devices and the increase in the pixel count bring about reduction in size of pixels to be incorporated into the devices. The reduction in size of the pixels worsens the light sensitivity of the devices, which is one of the basic requirements for the devices, and makes it difficult to capture clear images in poorly illuminated conditions. Therefore, it is important to improve the light sensitivity of each pixel and manufacture the devices at reduced cost by means of a process suitable for mass production.
As a technique for increasing the light sensitivity of the devices, the following techniques are known. For example, a technique of forming a microlens made of an organic polymeric material on a color filter is disclosed in Japanese Unexamined Patent Publication No. HEI 4(1992)-12568, and another technique of forming a lens inside a laminated structure between a light-receiving section and a color filter, which is so-called an intralayer lens, is disclosed in Japanese Unexamined Patent Publication No. HEI 11(1999)-87672.
As shown in FIG. 3, the solid-state image pickup device including such an intralayer lens comprises a CCD transfer channel 24, a reading gate section 23, a photoelectric conversion section (light-receiving section) 22 and a channel stopper 25 in a surface of a semiconductor substrate 21, and a transfer electrode 27 formed on the CCD transfer channel 24 via an insulating film 26 and a light-shielding film 29 formed on the transfer electrode 27 via an interlayer dielectric 28. The device further comprises a planarizing film 30 made of Boro-Phosphosilicate Glass (BPSG) or the like, an intralayer lens 74, a color filter 12 of R, G or B, a protective film 13 and a microlens 14. These are laminated on the light-shielding film 29 in this order. Here, the microlens 14 is placed above the photoelectric conversion section 22.
A method of forming the intralayer lens will be hereinafter described.
As shown in FIG. 4(a), implantation or the like of required impurities into a semiconductor substrate 21 is performed to form a photoelectric conversion section 22, a reading gate section 23, a CCD transfer channel 24 and a channel stopper 25. Subsequently, an insulating film 26 is formed on a surface of the semiconductor substrate 21, and then a transfer electrode 27 (whose film thickness is, for example, 300 nm) having a predetermined pattern is formed on the insulating film 26. Thereafter, a light-shielding film 29 (whose film thickness is, for example, 200 nm) is formed on the transfer electrode 27 via an interlayer dielectric 28 so that it covers the transfer electrode 27 and has an opening above the photoelectric conversion section 22.
As shown in FIG. 4(b), a BPSG film where each predetermined concentration of phosphorous and boron is predetermined is deposited on the light-shielding film 29 and semiconductor substrate 21 by an atmospheric pressure CVD process so as to have a film thickness of 600 nm, and is subjected to a reflow process at a temperature of 900° C. or higher, thereby to form a planarizing film 30 having a concave portion above the photoelectric conversion section 22. The surface of this concave portion constitutes a convex surface of an intralayer lens to be formed on this film 30.
As shown in FIG. 4(c), a silicon-nitride-series film 71 is formed on the planarizing film 30 by an atmospheric pressure CVD process. The thickness of the silicon-nitride-series film 71 is larger than the depth of the concave portion of the planarizing film 30. Since the concave portion is due to the shape of the light-shielding film 29 on the transfer electrode 27, the depth of the portion is about 500 nm. Thus, the thickness of the silicon-nitride-series film 71 is set to 500 nm or greater, for example, about 1500 nm.
Subsequently, a Spin on Glass (SOG) film precursor is spin-coated onto the silicon-nitride-series film 71 so that the SOG film has a thickness of 1500 nm, and is subjected to an annealing treatment (for example, at 400° C. for 30 hours), thereby to form an SOG film 72 having a flat surface.
As shown in FIG. 4(d), the SOG film 72 and silicon-nitride-series film 71 are etched back so that the SOG film 72 is completely removed and the surface of the silicon-nitride-series film 71 is planarized. In this process, the films above are etched so that a thickness of about 3000 nm in terms of an SOG film is etched. Etching back conditions are set up so that the etching selectivity of the SOG film 72 to the silicon-nitride-series film 71 is from 1.0:0.9 to 1.0:1.1. The above process can be performed by either of a plasma etching method such as a high-frequency parallel plates method, a magnetron high-frequency plasma method, a microwave plasma method or a magnetic field microwave discharge method.
Then, as shown in FIG. 4(e), an intralayer lens 74 formed from the silicon-nitride-series film 71 is formed on the planarizing film 30.
The aforesaid formation method of the intralayer lens has the following problems 1 to 3.    1. The silicon-nitride-series film 71 constituting the intralayer lens needs to be formed so that the thickness of the film 71 becomes larger than the depth of the concave portion of the underlying planarizing film 30. When the silicon-nitride-series film 71 having a thickness of 1000 nm or greater is formed by an atmospheric pressure CVD apparatus or a plasma CVD apparatus which is generally used in the formation of silicon-nitride-series film, the film 71 is plastically deformed by a film stress, and a crack occurs in the film. Furthermore, the film thickness of the film 71 is not likely to be uniform, which leads to nonuniformity of the thickness of the intralayer lens. As a result, the image quality of the solid-state image pickup device is deteriorated.    2. The SOG film 72 needs to be formed to have such a thickness that the concave portion of the underlying. silicon-nitride-series film 71 is filled with the SOG film 72 and the surface of the SOG film 72 becomes flat. When the SOG film 72 having a thickness of 1000 nm or greater is formed by spin-coating the precursor onto the film 71, followed by annealing, the film 72 is cracked and exfoliated by a film stress. The crack and exfoliation causes a defect in the shape of the intralayer lens. The defect is problematic to the image pickup device.    3. The SOG film 72 and silicon-nitride-series film 71 to be etched need to have a total thickness of about 3000 nm for the above-mentioned etching back process, and the etching selectivity of the SOG film 72 to the silicon-nitride-series film 71 needs to be set to about 1:1. The amount of etching is liable to vary in one wafer or wafers. Therefore, it is very difficult to form an intralayer lens having a uniform thickness. Further, etching apparatuses used in this process are expensive, and maintenance comprising control of the etching rate, dust control and the like takes a lot of labor.